Inducing Polarity in a Non-Polar Material

The growth of non-polar Cr₂O₃ on oppositely poled ZnO surfaces was characterized to determine how the polar substrate influences the properties of the non-polar film. The growth is 2D; however, the films appear initially disordered on both substrates. With increasing film thickness, the films ordered with a well-defined epitaxial relationship. The HRTEM results for thicker films confirm a clear interface and well-defined lattice structure near the interface and throughout the film, indicating that above a critical thickness the entire film reorganizes into an ordered structure. The polar interfaces show a small but noticeable band offset that decayed with increasing film thickness. Statistical analysis of UPS valence band spectra revealed an enhanced density of states near the Fermi level for Cr₂O₃ on the positive surface consistent with stabilization of the positive interface by charge transfer; in contrast, no significant valence band differences were observed between bulk Cr₂O₃ and thin Cr₂O₃ thin layers on the negative surface. To determine if the interfacial properties are sensitive to how the interface is formed, the polarity of ZnO films grown on ZnO (0001) and ZnO (000 ) with variable thickness Cr₂O₃ intermediate layers (ZnO/Cr₂O₃/ZnO) has also been studied. It was observed that the polarity of ZnO films grown on ZnO (0001) switched from positive to negative as the Cr₂O₃ intermediate layer thickened, while the polarity of ZnO films grown on ZnO (000  ) remained negative regardless of the Cr₂O₃ intermediate layer thickness. These findings highlight the possibility to manipulate non-polar thin film surface properties with a polar substrate.